In a typical wired or wireless network each device in the network will have its own clock to regulate the signals it transmits or receives. And in many networks a clock in a given device will operate independently of the clocks in each of the other devices, meaning that the phase of any given clock will be effectively random with respect to the phase of any other clock in-the network. In such a network it may be necessary, therefore, for a receiver to synchronize its clock with the clock of a transmitter before it can properly process data received from the transmitter. This process of phase synchronization is typically referred to as signal acquisition.
Ideally, each compatible device will have the same clock frequency, meaning that once the phase of the receiver clock is synchronized to the phase of the transmitter clock in an acquisition process, the receiver clock should be able to stay at this acquired phase for the remainder of the transmitted signal. However, this is generally not the case.
Although two compatible devices will ostensibly have the same clock frequency, the physical realities of circuit design will result in slight differences in clock frequencies. For example, clock crystals used for timing have a frequency tolerance in terms of parts per million (PPM) of their frequency. For example, a 10 MHz clock with a tolerance of ±100 PPM would actually have a clock frequency that was between 9.999 MHz and 10.001 MHz, or a possible difference of ±1 kHz. As a result, even though each device in a network may have the same model of clock crystal, these slight variations in the individual clock crystals may give each device a slightly different clock frequency.
And since a difference in frequency represents a systematical change in phase, if the transmitter clock and the receiver clock have slightly different frequencies, their clock phases will constantly drift apart, even once they are synchronized. As a result, many systems also employ a tracking process whereby after acquisition is complete, a receiver will monitor the phase of an incoming signal and periodically adjust the phase of the receiver clock to maintain its synchronization with the transmitter clock.
Of course, the greater the separation between the frequencies of the transmitter clock and the receiver clock, the more difficult this tracking process will be. Large frequency differences may increase the number of lost connections that occur when the tracking process fails and the phase of the receiver clock slips too far from the phase of the transmitter clock. Also, the receiver has implementation-dependent limitations that cap the size of the frequency difference that is tolerable. As frequency offsets approach this limit, noisy data will have a more pronounced impact on system performance. Data that would have been successfully detected with smaller frequency offsets will fail detection at larger offsets.
The tracking process would be simplified if the receiver had some information as to what the frequency difference (sometimes called the frequency offset or the frequency difference) was between the transmitter clock and the receiver clock. This information would allow the receiver to predict how the phase difference between the received signal and the local signal would change over time. And such predictability in phase change would provide for a larger allowable frequency difference between the transmitter clock and the receiver clock, while retaining the same level of system performance.
A larger allowable frequency difference between a transmitter and a receiver would allow devices to use clock crystals with a larger frequency tolerance. And since frequency tolerance and price are linked (the smaller the tolerance, the higher the price), this would allow the devices to use cheaper clock crystals, thus reducing device cost. As a result, it would be desirable to provide a method or circuit that would provide an estimate of the frequency difference between an incoming signal received from a transmitting device and a local signal generated by a receiving device.